1. Field of Invention
The present invention relates mainly to delta modulation communication systems. It does, however, have applications in other technical fields, e.g.: source encoding for data compacting and fingerprint extraction from pictorial or graphical data, etc.
2. Description of the Prior Art
In known delta modulation methods, an encoder forms a pulse if the signal being encoded is greater than an estimate of the signal value, and no pulse if the signal being encoded is less than the estimate.
Considerable effort has been expended in delta modulation systems for both video and voice communications. Some of this effort has been directed to reducing the number of bits or pulses required to describe the analog signal, as exemplified by U.S. Pat. Nos. 3,593,141; 3,824,590; and 3,339,142. Other systems, as exemplified by U.S. Pat. No. 3,643,180, have attempted to provide a higher signal-to-noise ratio.
Other efforts have been directed to adaptive-to-linear delta modulated signal converters, as exemplified by U.S. Pat. No. 3,703,688; conference circuits for delta modulated digital telecommunications systems, as exemplified by U.S. Pat. No. 3,842,351; and statistical delta modulation systems as exemplified by U.S. Pat. No. 3,393,364.
Adaptive delta modulators, as exemplified by U.S. Pat. Nos. 3,824,590; 3,652,957; and 3,339,142; have attempted to generate estimates approximating abrupt large amplitude voltage changes in the input signals. An alternative to adaptive delta modulations in approximating a signal with sharp rise times is disclosed in U.S. Pat. No. 3,723,909.
Hence, prior art efforts were directed to improving the operation of delta modulators in generating estimates to accurately follow abrupt voltage discontinuities in the input signal. However, when the input signal did not substantially change, such as during intervals of substantially constant levels of a video input signal, adaptive delta modulation systems had the undesirable effect on video signals of creating high granularity in the reconstructed video signal. This occurred because minor signal level fluctuations were estimated to be the beginning of an abrupt change, causing pulses to be formed when no significant change was present, and because, inherently, the conventional delta modulators, being differential devices, have to send error pulses even when no change in the signal is detected.
So far as is known, systems which adaptively and accurately followed abrupt changes in the input signal being encoded have been generally undesirable for tracking substantially constant video signal levels due to granularity in the reconstructed signal. Conversly, systems with low granularity in the reconstructed signal have been inaccurate and slow in responding to abrupt changes in the input signal. To overcome such undesirable effects, "adaptive" delta modulation systems have been proposed that try to reconciliate between the above conflicting features. They do so by changing the size (in the Estimator) of the increment (step size) in accordance with some nonlinear function of the past history of the differences in value between the signal and its estimate.
However, so far as is known, none of these systems can reduce granularity to zero. This is due, as already mentioned, to their intrinsic differential structure that requires the transmission of error signals even when the signal to be tracked is constant over long periods of time.
Moreover, in addition to such deficiencies, the nonlinearities introduced into these systems lead to unwanted large overshoots in the estimates of the signals they try to acquire and track. Such overshoots when introduced into estimates of signals bearing pictorial information will ultimately produce "multiple-edge" disturbances in the reproduced pictures.
Overshoot-suppression algorithms and the circuits developed to implement them seem to be not very effective in overcoming such deficiencies when introduced into conventional delta modulator systems. The modulation technique developed for the present invention, however, proved to be a significant improvement over this deficiency. This improved behavior is due to the fact that in the new (three-state) device the signal estimate, when recovering from an overshoot, has three rather than two states into which it can settle.